Effects of chronic growth hormone excess on cardiac contractility and myosin phenotype in the rat.

The effects of chronic growth hormone (GH) hypersecretion on intrinsic contractile properties of the myocardium were studied in rats bearing a GH-secreting tumour. Body weight and heart weight increased, but no true cardiac hypertrophy was observed. The maximum active force of left ventricular papillary muscle was increased, and the maximum shortening velocity of the unloaded muscle was unaltered. This was despite a marked shift of the myosin isoforms towards the low ATPase activity V3 form, which was associated with a similar shift of myosin mRNAs. Total ATPase activity, measured on frozen sections, was unaltered suggesting, together with the results of the mechanical study, an increase in the number of active enzymatic sites. Studies performed on skinned fibres confirmed the increased myocardial contractility, thus suggesting that myocardial adaptation to chronic GH excess occurred, at least in part, at the level of the myofibrillar contractile apparatus.